1. Field of the Invention
The invention relates to a superconductive electric direct current cable.
Such a cable is disclosed in WO 2008/148390 A1
2. Description of Related Art
In today's technology, superconductive cables include electric conductors of a composite material which includes ceramic material that changes into the superconductive state when the temperature is sufficiently low. The electric direct current resistance of an appropriately constructed conductor is zero when cooled appropriately, as long as a certain current strength is not exceeded. Suitable ceramic materials are, for example, BSCCO (Bismuth-Strontium-Calcium-Copper-Oxide) as material of the first generation, or ReBCO (Rare-earth-Barium-Copper-Oxide), particularly YBCO (Yttrium-Barium-Copper-Oxide), as material of the second generation. Sufficiently low temperatures for bringing these types of materials into the superconductive state are between 67 K and 90 K. Suitable cooling agents are, for example, Nitrogen, Helium, Neon and Hydrogen or mixtures of these materials.
A direct current cable, in the following called “cable”, as they are described hereinabove, may, for example, be used as a current supply cable on ships or as connecting cable between points of intersection within a transmission network as well as a connecting cable between different transmission networks. The cable can be, for example, an arrangement for a high-voltage direct current transmission (HVTCT), which can be used for bridging longer distances, for example, as sea cable. Compared to superconductive alternating current cables, superconductive direct current cables have the advantage that there are no electric direct current losses. In addition, the direct current cables are not burdened with charging currents and there are no undesired inductive voltage drops.
The above-mentioned WO 2008/148390 A1 discloses a two-phase electric cable for the current supply of users of electrical current. The cable is constructed as a superconductive cable with two-phase conductors which are combined with each other to form a unit by an inner dielectric and are combined into unit by an inner dielectric from each other. The cable is arranged in a cryostat which is composed of two concentric tubes between which a vacuum insulation is arranged. The cooling agent for producing the superconductive state of the phase conductors is conducted through the cryostat. The cooling of such a cable is problematic because the dielectric constitutes a thermal insulation for the cooling of the superconductive phase conductor.